1. Field of the Invention
The present invention relates to a switching control circuit for a field controlled thyristor and in particular concerns a field controlled thyristor control circuit which is capable of switching on and off a large load current with a relatively small control signal.
2. Description of the Prior Art
The field controlled thyristor is a semiconductor switching element including a semiconductor region of one conductivity type (referred to as the gate region) formed locally in one of the regions of a pn-junction diode that has the other conductivity type and a gate electrode electrically contacted to the gate region.
In the field controlled thyristor of the structure described above, when the junction between the region of the other conductivity type and the gate region is biased backwardly, the forward current flow through the pn-junction diode is cut off (turn-off) due to a depletion layer formed in the region of the other conductivity type. On the other hand, when the backward bias is removed, the depletion layer will disappear thereby to allow the forward current to flow (turn-on). In this manner, the field controlled thyristor exhibits a switching function. As compared with the hitherto known semiconductor switching elements such as transistors, thyristors or the like, the field controlled thyristor has a significantly reduced turn-on time and additionally enjoys an enhanced di/dt capability upon turn-on operation. Further, means for turning off the field controlled thyristor can be implemented in a much improved manner.
FIG. 1 shows a phase relationship between an anode or load current iA and a gate voltage V.sub.G of a field controlled thyristor (hereinafter referred to also as FCT in abridgement). When a backward bias voltage is applied to the gate electrode of the FCT, e.g. a negative gate voltage -V.sub.G is applied in the case of the FCT having a P-type gate region, no anode current can flow through the FCT provided that the absolute value of the gate voltage .vertline.V.sub.G .vertline. is not smaller than a predetermined value. When the gate voltage .vertline.V.sub.G .vertline. is lowered below the predetermined value, the anode current is allowed to flow and continues to flow unless the negative gate voltage of the prescribed value is applied. Upon application of the negative gate voltage, the anode current will be turned off again.
Typical examples of such an FCT as described above are disclosed in the specifications of U.S. Pat. Nos. 4,037,245 and 4,060,821, for example.
As a method for controlling an FCT, it is conceivable to connect a transistor in series to a backward bias voltage source, as is exemplarily shown in FIG. 2. On the assumption that the FCT concerned has a gate region of P-conductivity type, the control circuit shown in FIG. 2 comprises a series connection of an npn-type transistor TR, a gate voltage source E.sub.G, a main power supply source E.sub.S for the field controlled thyristor FCT and a load R.sub.L which are connected between the gate and the anode electrode of FCT as well as a series connection of a base current supply source E.sub.B, and a switch S.sub.B inserted between the cathode electrode of FCT and the base electrode of the transistor TR. Operation of this circuit is graphically illustrated in FIG. 3. As will be readily understood, when the switch S.sub.B is turned on, the transistor TR becomes conductive due to the base current iB supplied from the base current supply source E.sub.B thereby to provide a low impedance state, as a result of which the gate voltage VG of the gate voltage source E.sub.G is applied between the gate and the cathode electrode of FCT to turn it off. On the other hand, when the switch S.sub.B is turned off, the transistor TR becomes non-conductive or off to inhibit the application of the gate source voltage VG between the gate electrode and the cathode electrode of FCT. Consequently, FCT is turned on. In this manner, in order to hold FCT in the non-conductive state continuously, it is required to supply constantly the base current to the transistor TR to maintain it in the conductive state. Accordingly, in the applications in which FCT is to be maintained in the off or non-conductive state for a relatively long duration by using a single-shot pulse current of a relatively short duration or pulse current of a relatively low repetition frequency, it becomes necessary to employ the base current source E.sub.B of a relatively large power capacity. Further, the base current to the transistor TR needs to be cut off for making FCT conductive for a relatively short duration. In this connection, it should be noted that the commercially available pulse generators are not generally suited to be used straightforwardly as the base current source in such applications.
Another disadvantage of the control circuit for FCT shown in FIG. 2 may be found in the fact that a transistor having a high over-current capability has to be used for the transistor TR because the anode current iA will flow through the gate circuit of FCT when the anode current iA is turned off. An example of experimental measurement of the anode current and the gate current is graphically illustrated in FIG. 4. As can be seen from this graph, the peak value of the gate current iG will sometimes attain a value substantially equal to the peak value of the anode current iA. It goes without saying that the correspondingly increased gate current iG will also flow through the transistor TR at the time when a large anode current of FCT is cut off. Consequently, a transistor having a high over-current capability is required for the transistor TR. In spite of the requirement described above, the transistors which are commercially available are generally poor in respect of the over-current capability, which provides a difficulty in implementing the control circuit shown in FIG. 2 for practical applications.